Automatic temperature control system



April 10, 1951 A. s. FlTZ GERALD 2,547,933

AUTOMATIC TEMPERATURE CONTROL SYSTEM Filed Dec. 12, 1946 4 Sheets-Sheet 1 All? mag E E3 00,4

INVENTOR flaw 5 572626440 BY An 1 1 M ATTORN EY Apnl 10, 1951 A. s. FlTZ GERALD AUTOMATIC TEMPERATURE CONTROL SYSTEM 4 Sheets-Sheet 2 Filed Dec. 12, 1946 A50 P5? CENT P? CENT 50 P67? Cf/VT OZ/fS/Df Alli. TEMPERA 70/?5 Tica.2.

INVENTOR i -*5 A M ATTORNEYS Apnl 10, 1951 A. s. FITZ GERALD AUTOMATIC TEMPERATURE CONTROL SYSTEM 4 Sheets-Sheet 3 Filed Dec. 12, 1946 0075/06 All? TEMPiiA Tl/FE I l I 50 40 30 mink); RQQ

INVENTOR m. m 6 w F Cd Y B T ials-a.

S Y E N R m T A April 1951 A. s. FlTZ GERALD 2,547,933

AUTOMATIC TEMPERATURE CONTROL SYSTEM Filed Dec. 12, 1946 4 Sheets-Sheet 4 H07 WA 75/? TEMPA'AA TUAE 00/15/05 A/l? TE'MPiP/i Til/FE AzA/v 5 57-262mm T MW' ATTO R N EYS Patented Apr. 10, 1951' AUTOMATIC TEMPERATURE EONTROL SY STEM Alan .S. Fitz Gerald, Wynnewood, Pa., assignor to Warre 'Webs e corporation of New Jersey r & C m any Gamde i,

Ann icatmmDe m e :946, =eria No- 3,115,855 7 Gla'in e- (01.236 31) This invention relate to electrical control sysems nd or par cul r y to ystems for can r iin a process 9. B i i s cha wa a the rate or intensity of the said process is conl e i a o dance "wit th ma n tu of a nhys cal quant ty- M inventi n i na t u arl ap ic ble t hea ing sy tems and especially to major heating syste s employed in .i build n s wh e n -sive radiationis installed.

My nvent o may w th d antaee b ca ied out with ma netic amp ifie a an em nts o h type ,I have des ribed in :my -pending appl tion, Serial No. 71,9Ai5, fil d D cem er -1. ,1945- It is an object of my invention to provide impro e means and fifihpiifi :iQr wa ring t amoun o been de ivered to a bu ldi g n a ,cordance with the temperature of the air outside Q thebnfldin It is another ob ect o m i ve t on to r r d improved means and methods for maintaining a predetermined relation between :the temperature of the-out a andt ia u rtz f hea d l ver d .to a building.

It is further object of ;my invention to pro vid ranesp eiallyrfleid le co ol s ste b m ans .of which the relation between the temperature of the air, and the heat delivered, :may be varied so as to select a plural ty of d fi ent sentro' pharacteristics.

It is yet another object ofmy invention to pro- ,vide means for adjusting the control characteristics in accordance with a pluralityof different circumstances, as for example, different contemr plated minimum temperatures of the outside air, or minimum temperatures of the heating vapor or fluid.

It is yet another object of my invention to vac- ,cornplish the above results with apparatus of an exceptionally reliable type, combining a high order of sensitivity with freedom from contacts, movin pa t or a r us requir ng main enance, attentiomor renewal.

These and other novel features which Ihelieve to be characteristic of my invention will be set forth with particularity in the appended claims. My invention itself, howevenhoth toitsorganization and method of operation, together with iurther objects and advantages thereof, mayhest be understood with reference to the following description taken in zcon ect on with th ac tam- ,P n ne d awi s i which:

vF g. i is a diagra t r n esen ationo a embodim nt my inv nt on;

Fig. *2 is a chart illustrating the manner of opera io o a a u e o the em odiment of my inr n enshown iti is an th r chart iin t in u the fe t o t e a t n o m inven o and Fi 4 is yet anothe ha i il ustr tin th mati ner O Op t pi'a further feature of my invent on Re errin t the dr w gs .1 sh wn li'i portion of a fluid ,or vapor heating system .I, in,- cluding a control valye 2. 'Ifhe yglve 2 is aetnte th o h a tem eraute reS Qn r-e contro .s tem c m ri n a Pai o th rma r iatapee elem nts 3R The e em 3 is m unted adjacent to or otherwise responsive to, thefltemra i =Q he heating m dium- The t e mal e em n 3R islmounte lat a u table locat n out: o -w eo s "so to b responsiv to t e am ien externa temn ram Aecdrd to m invent on t e hea i g me ium ism-al e :m ntaii1 sia a empera e wh ch i di ert re ati n wi h th m i n tem e ature.

y cont o ,s stem m t b d e tirrci ts L AR each ompris ng the e nial eih 31 and tti e the resisto of fixed T e brid es A AR a e ergi d a r-om a a so ce 1. th ou h 3 an so ing .transt mea :fiL ha in primary win in s 1 5 R:-1 n sec da ywin ings 1th ,1 IR.

frhe t o primar wind ngs 0L, ,IQR as ,S QW in-the draw g a e connected in e esa p e th aourc l9,- lrel p tomca a ros 1th Qll i L on o i e 2 i rman ali Ya ing therel at r le ai a i of the ansl o mer 3 L 3B and a c d ngl th rela i e len g z t n @Q th br dg 41 AR li e se The tqntm vic 1 co p i s a variab e inductan e s uctu ,sisnn two-s eneid hay n nio n t lueaer 4L impartially .B tBr n iltocofls 15L, HR together with manually operable means suc ht a's a rael; liandapinion I].

The i tent on sb im en Milt iii-Ree 451:, B .resnectivelyarei onnee t atqeeth r- It will be apparent :to thos sk led in th that when :the two plungers 14L, 515R are immidposition, that [is to say, equally entrant into the windings ISL, 15R, :the voltages across 15L, 15R will -be equal as will also be the-voltages across the-primarywindings-N1 j It will be evident that if the plungers ML, MR are moved towards the "left so that ldR enters fu her in t th .,s.=Qi1 a m and th Pl ge M the. bridge is f lR goes out of' "ture, if desired, the element 3L may be made of which obtains jjvention will be such that both bridges will be- ,come unbalanced at the same time with increase "of resistance in'one'arm.

is retracted from the coil iL, the impedance of. I5R will increase and that of [5L will decrease. Accordingly the voltage across HIR will increase and that across liiL will decrease. Thus the energization of the bridge 4R will be augmented and that of the bridge 4L will be reduced.

AS shown in Fig. 1 the bridges 4L, 4R are energized across their horizontal diagonals with rectified current from the rectifiers 8L, SE. The bridges 4L, 4B are connected in series through their vertical diagonals by means of conductors I8L, i8R, and i9, and through the input 32L32R of a magnetic amplifier 35 as referred to hereinafter If the resistors 5L, 5R, 6L, 6R be considered to be bridge ratio arms of equal valve, the value of the resistance in the arms-adjacent to thethermal elements 3L, 3R, that is to say the arms in which resistor 1L, IR are situated, may be adjusted so that the bridges are in balance under temperature conditions such that no heating control action is desired. This might be the case if the ambient temperature out-of-doors were about 70, as might also be the temperature of the fluid in the heating system I.

' Thus the bridges, iL, 4R may be considered to be inbalance at 70 F. at which condition there will be no E. M. F. existing between conductors I8R and i9; nor between I8L and I9. Should the weather become "colder the bridge 4R will no longer remain in balance and there will appear between conductors 18R and IS a direct current voltage which will be in accordance with the drop in outdoor temperature.

' For the purpose of explanation of the operationof my .invention it willbe assumed that when thebridge 4R goees out of balance, due to fall of temperature, conductor 18R becomes pos'i jtive with reference to conductor l9. It will be apparent "to those skilled in the art that this will depend upon-the manner of connection of the rectifier SE to the bridge 4R, that is to'say, whether the right hand or lefthand diagonal of "made positive or negative; and in addition whether-the temperature coemcie'nt of the thermal elements 3R is-positive or negative.

inventionmay-be carried into effect with j'thermal elements "3L; 3R made of a material of "either a positive or negative-temperature co- "efiicient.

Since the bridge '4L' goes out of baltemperature and the bridge ance with increasing e with decreasing temperabalanc a material hav ng a positive coefficient, and the element 3R may be made of a material having a negative coefiicient. advantage that both of This selection has the the bridges go out of balance congruently, that is to say,- the action durin the'operation of my in- Under these conditions the characteristics shown in the charts of Figs. 2, 3, and-4 will be straight lines; If both of the elements 3L and 3R have like temeprature coefficients there will be a slight tendency towards curvature of the linesindicatin characteristics in Figs. 2, 3, and 54. However, this tendency towards curvature .of:the characteristics can be minimized by maintaining a suitable minimum value for the resista-nces-20L and-20R (as hereinafter described). In like manner'the bridgedL may be adjusted .:':;to ba c a t- .0? The h m l e n will normally be subjected'to temperatures rising above F. Accordingly, the connections between the bridge 4L and the rectifier 8L are made so that when the bridge becomes unbalanced due to increase of water temperature the conductor ISL becomes positive in relation to the conductor 19.

The bridges 4L, 4R, as to nals, are connected by the and 19, in opposition.

In addition I shown in resistors 20L, 20R, and 2|. for the purpose of adjusting of my invention so as to give a plurality of different operating characteristics at will. As shown in the diagram the resistor Z! is connected in the adjacent'arm of the water bridge 4L, in series with fixed resistor TL. Means are provided for selecting a plurality of difierent values of the resistors 2|, ML and 2013. as for example by means of movable links indicated at 22, 23, and 24, each of which may be placed in one of several positions. For example, I have shown in Fig. 1, an arrangement for selecting any one of four different resistance values for the resistor 25. I have a similar arrangement in respect of the resistor 20R. As will be explained in greater detail hereinafter the adjustment of 20R is to be coordinated with that of 2|. I prefer that this should be accomplished by means of some form of insulating mechanical arrangement connecting 22 with 24 such as I have indicated schematically by the broken line at 25.

I show also in Fig. 1 an arrangement for selecting any one of five different resistance values for the resistor 20L. The several positions of the links '22, 23, 24 are indicated on the drawing by the legends 70, and and 20, '-10, 0, +10, +20 which have reference to the performance characteristics desired as will be hereinafter explained in greater detail.

Conductors l8L, 18B. are connected to a magnetic amplifier 35 for controlling the valve 2, by means of an arrangement which may be substantially similar to that which I have shown in Fig. 6 of my co-pending application Serial No. 719,445 to which reference may be had for further detailed description.

In brief the magnetic amplifier 35 has two binding posts 32L, 32R for receiving an input of reversible direct current polarity. The magnetic amplifier 35 also has two pair of output terminals 34L, 34R for supplying amplified direct current to saturable core devices having cores IOOL, 7 MB, A. C, windings ZililL, 208R and D. C. saturating windings 3t 13L, 369R to which binding posts 34L, 34B are connected.

The valve 2 of a hot water heating line i is ac- .tuated by a motor which may be of the induction type having a rotor 42, windings 43L, iBR andja capacitorfl i, energized from the A. C. source' 'l3 all as described in my co-pending application S e- ,rial No. 719,445. As therein explained when D. C. input is applied to 35 of polarity such that 32R is positive and 32L is negative current is delivered by 34R to the saturating winding tiiEiR but no appreciable current isdelivered by 34L to 39DL under which condition the motor turns in one direction. In Fig. 1, under this condition the motor should operate in the direction which increases the amount of heat delivered; j z Conversely ifBZL isp'josi'tive'with regard to 32R the motor will operate in the reverse direction, that is tosay, less vheat will be furnished to the heating system.

their vertical diagoconductors 18L, I8R

Fig. 1, three tapped These resistors are the control action accuses From the above description it will'be clear that when the right hand bridge 4R, in Fig. 1, goes out of balance the efiect is to caus the temperature to be raised. In like mannerwhen the left hand bridge 4L goes out of balance there is set up control action causing the water temperature to be lowered.

We may-now consider the conjoint action of the. two bridges. If the temperature is exactly-70 F. both bridges, by hypothesis are balanced and no voltage appears between 1 9 and either 18L or 18R; nor is there any energization of impact terminals 32L, 32R of magnetic amplifier 35.

If the outdoor temperature drops the air bridge 4R will become unbalanced. This will cause the motor to open up the valve 2 and increase the'water temperature. Asa result ofthis the water bridge 4L will also become unbalanced. Since the air and water bridgesl-L and 4R are in opposition as soon as the out-of-balance voltage appearing between i9 and 18L equals that which exists between H! and IBR, 32L and 321% will again be de-energized and the motor will come to rest.

his therefore apparent thatforany given temperature of the outside-air there 'will be a corresponding temperatu-re of the Water in the heating system which will cause the system-either to-come into a condition ofstatic balance, or to fluctuate within narrow limits,

This condition may be illustrated by the chart in Fig. 2. For the purpose of explanation in reference to Fig. 2 it is assumed that link 22 and 25 are in the position indicated in .Fig. 1 by the legend tion there is no additional resistance added in the arm of the water bridg 4L in which the resistor IL is situated. Under this conditionAL isadjusted for perfect balance when the water temperature is 70. In Fig. 1, Ishow the link .23 in the position indicated bythe legend I)". This is the position in'which the link 23 :is to beJplaced .ifit .is desired that the maximum prescribed volume of heat in the heating system is to be delivered if the outdoor temperature-rallsto zero degrees .F.

For example, let it :be assumed'that themaximum desired water temperature is 200 F.

Accordingly in practicing :my invention, to secure this operating :condition, the control device 12 is set in mid-position, which position may be designated by the legend -10.0- %,'t o indicate that normal controlling action is :desired. The value of the resistor 28L, at the tap marked zero, is then vset so that when the water temperature is 200 the 'out-of-balance voltage set up by the water i bridge 4L, betweenconductors 18L and :I9 is exactly equal to the out-of-balance voltage set up by the air bridge' lR-between conductor 18R and I9 when the out-of -door temperature is zero.

The energization of the air bridge under this condition, the link 24 ,as stated, being in theposition marked 70, is adjusted to the correct amount :by setting the resistance of 203 with link 24 so placed, at the proper value.

Referring again to Fig. 2 I have shown :therein a point marked 0, which is at the intersection of the axes correspondingrespectively to an outdoor air temperature of zero and a water temperature of .200". The middle of the three linesshown in Fig. 2 is drawn from the axis of balance, that is to say, 70 air temperature and 70 water temperature, through this point. This line indicates the control characteristic 'of my invention. For any given outdoor air temperature the correisponding hot water temperature-'at'whichthe two It will be noted that under this condi- 6 bridges will balance, and at which the motor will come to rest, can-be read off.

I also .show in Fig. '2 the efiect of adjusting the control device 12.

For example, if the weather condition is such that high winds are likely, whereby the heat dissipation of a building may be increased, it may be desirable that heat be delivered, for anygiven outside air temperature, at a somewhat greater rate than that indicated by the middle line in Fig. '2. Accordingly, I may adjust the control device l2 so that more than per cent of the normal heating action is given. For "example, I may turn the control 12- to the extent indicated in the figure by the legend thereby retracting the plunger ML from the coil l-5L and displacing MR correspondingly so that the voltage applied to the primary windings [0 are no longer equal, the transformer 53R being energized to a greater extent than 9L. This will energize the air bridge more and the water bridge less. Since the effect of the air bridge is to increase the heat and that of the water bridge is to reduce the heat, this will, for any given outdoor temperature produce a controlling action so as to maintain the water temperature at a higher value than that indicated by the middle line in Fig. 2.

Since however, irrespective of the relative cnergization of the two bridges, neither of them produces any out-of-balance voltage at '70", the operation of 12 does not effect the intersection of the control characteristic, as indicated by the lines in Fig. '2, with the normal temperature axis; that is to say, the lower left hand corner of the chart in Fig. 2 corresponding to 70 both air and water temperature.

Thus the action'of the control device 12 is to swing the line representing the control charac teristic about the normal temperature axis. I have therefore shown on Fig. 2- the characteristic that results with the control device I2 set to the .150 percent corresponding to an increase in delivered'heat of '50 per cent above normal.

Converselyif the device 12 be moved in the opposite direction the eitect is to deliver less than the normal amount of heat for any given air temperature. Accordingly I have shown in Fig. '2 a third line marked 50% which shows the control characteristic when the device i2 is so positioned.

The control device may desirably be adjusted to give less than 100 per cent heating action in the absence of wind and when the building to be heated is exposed to directysunlig'ht.

The effect of the adjustment of the value of the resistor 26L, by shifting the link 23 is indicated in Fig. .3 in which, in similar manner, the relation between the "water temperature maintained, and the outdoor temperature, is shown.

Again since the water bridge AL is in balance at 70, "variation in the extent to which EL is energized produces no change in the characteristic at 70. However, if the -energization of the water bridge 4L'be decreased, as by increasing the value of 20L, in order that the out-of-balance voltage of 4Lmay be equal to any given outof-balance voltage of 4B,,the water bridge must be unbalanced to an additional extent. In other tion of the water bridge {4L "should be increased byreduc'ing the value ofthe ires'istor 20L.

Accordingly, the five "lines shown in Fig. 3 are desired result.

drawn through the intersection of the 200 axis with, respectively, the 20, -10. 0, +10. +20 values of the air temperature. Thus each of these five lines shows the characteristic of the control action that will be given for each of the five correspondingly designated positions of the link 23.

The characteristics shown in Fig. 3 refer to the one hundred per cent setting of the control device l2.

It is to be pointed out that this control action produces substantially the same result as does adjustment of I2. However, it is desirable, when installing equipment according to my invention, to select one of these five positions for a permanent setting according to the climatic zone in which the apparatus is situated. It will be noted that the third or middle line shown in Fig. 3 is identical with the middle line shown in Fig. 2.

For any of the five positions of the link 23 more or less than the normal or 100 per cent control action, may be secured by adjustment of the control device E2. in other words a control action corresponding to a displacement of any one of the five characteristics indicated in Fig. 3, clockwise or counter-clockwise, pivoting at the normal temperature axis of '70, may be produced by operation of the device I2.

I show in Fig. 4 additional modified control characteristics which may be carried into effect by adjustment provided and hereinbefore described, in reference to the resistors 20R and 2|. As stated, the characteristics indicated in Fig. 2 and Fig 3 are obtained if the links 22 and 24 are placed in the positions indicated by the legend 70. Under this condition the water bridge AL .is in balance at 70. In Fig. 4 I show at (a) in part, the five control characteristics indicated in Fig. 3, that is to say, the characteristics furnished with the links 22, 2 3 at the position marked 70 omitting the upper portion where intersection would create confusion.

Under certain heating system conditions it may be desired to operate with a higher water temperature base, as for example, 100 instead of 70; that is to say, it may be desired to maintain a water temperature of 100 F. when the out-door temperature is 70. According to my invention this result may be accomplished by adjusting the resistance 21 so that the water bridge AL is in balance at 100 instead of at 70. Assuming that the thermal element 3L has a positive temperature coefilcient this will require the addition of resistance in the conjugate arm of AL in which the resistor FL is situated, since the resistance of 3L will be greater at 100 than at 70. Accordingly the resistance of the whole of the resistor 2 I, which is utilized with the link 22 at the position marked 100, is given the appropriate value.

This alone will not accomplish completely the If the bridge is adjusted to balance at 100 instead of 70, at some higher value of temperature, as for example, 200 F., the bridge 4L will not be as much out-of-balance as will be the case, at the same temperature, if it were balanced at 70 instead of 100.

Accordingly for any given water temperature,

the out-of-balance voltage set up by the bridge 4L between the conductors I8L and [9 will be less, with the bridge set to balance at 100, than would be the case if balanced at 70. Thus to maintain any desired water temperature it is necessary to modify the relation between the energization of the two bridges 4L, 4R. Since the out-of-balance voltage set up by 4L, resulting Letters Patent of the United States is:

from any given water temperature, has been reduced, it is necessary to reduce the out-of-balance voltage set up by 4R if the out-of-balance voltages of the two bridges are to remain equal.

Accordingly when changing link 22 from the 70 position to the 100 position, link 24 at the same time is-to be changed so as to increase the resistance of 20R and reduce the energization of the air bridge 4R to the corresponding necessary extent.

The full value of 20R, utilized when link 24 is in the position marked 100, is therefore adjusted to a value such. that the two bridges are in balance at 230 F. under this condition.

The effect of this adjustment is to add 30 to all of the characteristics shown in Fig. 3. In other words the group of characteristic curves is displaced vertically by this amount exactly as shown at (d) in the upper portion of Fig. 4.

As stated in respect to Fig. 3 any one of these characteristics can be further modified for increased or decreased heat by action of the control device I2.

I show also in Fig. 1 two intermediate taps on 2| and 20R, correspondingly marked 80 and If the links 22 and 24 are placed in the 80 position all of the curves shown in Fig. 3 are displaced upwards corresponding to an increase of temperature of 10 throughout the operating range. Correspondingly, in the position marked 90 a vertical displacement of 20 is provided. The operation which results from the 80 and the 90 position is indicated fragmentarily at (b) and (c) in Fig. 4, the portions of the characteristics which would cause confusion by intersection being omitted.

It is to be understood that the five lines radiat ing from the normal temperature axis of 70 air temperature and water temperature, shown in part at (a) in Fig. 4, are exactly the same as in Fig. 3. That is to say, they intersect the five air temperature axis of 20, -10, 0, +10, +20 at 200. With the links 22 and 24 in the 80 position, the five lines in Fig. 4, shown in part at (b) their origin being 80 water temperature, would intersect the corresponding air temperature axis at Likewise, with a 90 water base the live lines shown at (c) intersect correspondingly at 220.

The characteristics for the water base temperature are fully indicated at (d) in Fig. 4, intersecting at 230 water temperature.

Although I have chosen a particular embodiment of my invention for the purpose of explanation, many modifications thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except in so far as is necessitated by the prior art and the spirit of the appended claims.

What I claim as new and desire to secure by 1. Apparatus for controlling the supply of a fluid heating medium in a conduit, comprising:- a valve for variably admitting the fluid to the conduit, electrical means for variably adjusting said valve, a first thermal resistance element responsive to the temperature at one location, a second thermal resistance element responsive to .the

temperature at another location, a first electrical bridge including said first resistance elementa second electrical bridge including said second resistance element, means for separately energizing each of said bridges across one conjugatearrn of each with electric energy, said energizing means comprising means for adjusting the value of the electric energy supplied to each'bridge, means connected to said energy adjusting means for concurrently increasing the energization of one of said bridges and decreasing the energizationof the other of. said bridges, means connecting the opposite conjugate arms of said bridges said control means, a first thermal resistance element responsive to the temperature at one locaation, a second thermal resistance element responsive to the temperature at another location, a first electrical bridge including said first resistance element, a second electrical bridge including said second resistance element, means for separately energizing each of said bridges across one conjugate arm of each bridge with electric energy, said energizing means comprising means for separately adjusting the value of energy supplied to each bridge, means connected to said energy adjusting means for concurrently increasing the energization of one of said bridges and decreasing the energization of the other of said bridges, and magnetic amplifier means connected to said opposite conjugate arms and to said electrical means for actuating said control means in response to the voltages across said last-mentioned arms.

3. Apparatus for controlling the supply of heat in accordance with a relation between the temperature at two difierent points, a first thermal resistance element responsive to the temperature at one of said points, a second thermal resistance element responsive to the temperature at the other of said points, a first electrical bridge including in one arm thereof said first thermal resistance element, a second electrical bridge including in one arm thereof said second thermal resistance element, a source of alternating current, a pair of rectifiers energized from said source, means for energizing each of said bridges across one conjugate arm of each bridge from said rectifiers, said energizing means comprising means for adjusting the energy supplied to one of said bridges, means connected to said energy adjusting means and to one arm of the other of said bridges for concurrently adjusting the energization of said one bridge and the balance of said other bridge, whereby the output voltages across the opposite conjugate arms of said bridges for a predetermined temperature differ ence between the temperatures at the said two different points may be changed while the output voltages for another temperature difference therebetween may be maintained equal, means connecting the opposite conjugate arms of said bridges so that the out-of-balance voltages de rived therefrom are in opposition and means energized in accordance with the difierence between said last-mentioned voltages for controlling the supply of heat.

4. A heat regulating system comprising: control means for varying the amount of heat delivered, electrical means for variably adjusting said control means, a first thermal resistance element responsive to the temperature at one location, a second thermal resistance element responsive to the temperature at another location,

.a first electrical bridge including said first resistance element, a second electrical bridge including said second resistance element, means for energizing each of said bridges across one conjugate arm of each bridge, said energizing means comprising means for adjusting the energy supplied to one of said bridges, means connected to said energy adjusting means and to one arm of the other of said bridges 'for concurrently adjusting the energization of said one bridge and the. balance of said other bridge, whereby the output voltages across the opposite conjugate arms of, said bridges for a predetermined temperature difference between the temperatures at the said two different locations may be changed 'while the output voltages for another temperature difierence therebetween may be maintained equal, and magnetic amplifier means connected to said opposite conjugate arms and to said electrical means for actuating said control meansin response to they voltages across said opposite conjugate arms.

5. In apparatus for controlling a supply of heat in accordance with a relation between the temperatures at two difierent points, a first thermal resistance element responsive to the temperature at one of said points, a second thermal resistance element responsive to the temperature at the other or" said points, a first electrical bridge including said first-mentioned thermal resistance element, a second electrical bridge including said second-mentioned thermal resistance element, a source of alternating current, a pair of variable inductance devices, means for concurrently and oppositely actuating said devices, means for connecting said devices in series across said source, a first rectifier energized in accordance with the voltage across one of said devices, a second rectifier energized in accordance with the voltage across the other of said devices, means for energizing said first bridge from said first rectifier and said second bridge from said second rectifier, whereby the relative energizations of said bridges is varied in accordance with the position of said actuating means, thereby to vary the rate of change of the temperature at one of said points in response to a temperature change at the other of said points.

6. In apparatus for controlling a supply of heat in accordance with a relation between the temperatures at two difierent points, a first thermal resistance element responsive to the temperature at one of said points, a second thermal resistance element responsive to the temperature at the other of said points, a first electrical bridge including said first-mentioned thermal resistance element, a second electrical bridge including said second-mentioned thermal resistance element, a source of alternating current, a pair of variable impedance devices, means for concurrently and oppositely actuating said devices, means connecting said devices in series across said source, means for energizing said first bridge in accordance with the voltage across one of said devices, means for energizing said second bridge in accordance with the voltage across the other of said devices, whereby the relative energizations of said bridges is varied in accordance with the position of said actuating means, thereby to vary the rate of supply of heat upon a temperature change at one of said points.

7. In apparatus for controlling a supply of heat in accordance with a relation between the temperatures at two difierent points, a first thermal resistance element responsive to the temperature at one of said points, a second thermal resistance element responsive to the temperature at the other of said points, a first electrical bridge including in one arm thereof said first-mentioned thermal resistance element, a second electrical bridge including in one arm thereof said secondmentioned thermal resistance element, a source of alternating current, a pair of rectifiers energized from said source, means for energizing each of said bridges across one conjugate arm of each from said rectifiers, respectively, means connecting the opposite conjugate arms of said bridges so that the out-of-balance voltages derive-d therefrom will be in opposition, means energized in accordance with the difference between said volt ages for controlling said supply of heat, together with an adjustable resistance connected in another arm of said first-mentioned bridge, a second adjustable resistance connected to vary the energization of said second-mentioned bridge, and means for jointly adjusting both of said adjustable resistances so as to provide a variation in said controlling action.

ALAN S. FI'IZ GERALD.

REFERENCES CITED UNITED STATES PATENTS Number Name Date 2,050,446 Meyer Aug. 11, 1936 2,250,946 Brown et a1. July 29, 1941 2,272,492 Weyher Feb, 10, 1942 2,297,705 Jehle Oct. 6, 1942 2,421,420 Hathaway June 3, 1947 2,431,790 Crosthwait et al. Dec. 2, 1947 FOREIGN PATENTS Number Country Date 45,078 France Mar. 11, 1935 777,297 France Nov. 26, 1934 

